Fig. 2. Developmental sequence of gene expression patterns in the dbd lineage. (A,B) Double labeling of gcm mRNA (purple) and Nubbin (orange) in the embryonic PNS (single abdominal segment; dorsal up, anterior to left). The dbd lineage is bracketed. The behavior of Nubbin-positive cells located dorsal to the dbd lineage (white circle) acts as a temporal indicator of embryogenesis. The Nubbin expression in this cell is absent by stage 13. We tentatively refer to the processes of the glial fate induction as stages I-IV. (stage I) Nubbin expression is initially detected in both daughter cells at the beginning of stage 12. The smaller daughter cell (arrow), located apicodorsally to the larger daughter cell differentiates as DBDG. The presumptive dbd neuron is indicated by an asterisk. (stage II) Nubbin becomes down-regulated in the presumptive glial cell prior to the onset of the gcm expression. (stage III) The expression of gcm is initiated in the smaller daughter cell where Nubbin expression is low (arrow). (stage IV) After gcm becomes highly activated, Nubbin is re-expressed in the glial daughter cell (arrow). Single labeling of Nubbin in the dbd lineage is shown in inset for stages III and IV. The dda neuron (out of focus) is indicated by an arrowhead. (B) At stage 16, Nubbin expression is again restricted to the dbd neuron. Double labeling of REPO (black) and Nubbin (orange) in the dbd lineage at stage 16 is shown in the inset. Strong REPO expression is detected in DBDG, in which gcm expression has already disappeared (arrow). The dda neuron is located dorsal to the dbd lineage (arrowhead). In this stage, Nubbin expression is detected in the dbd neuron, dda neuron and ligament cells within the PNS. (C) Summary of the expression dynamics of gcm mRNA and Nubbin in the dbd lineage. g, glial cell; n, neuron.